fix(frontend): the layer gates cover what the move left outside them, and each page keeps its own logic

Gates the folder move had narrowed or left open: the MenuCheck contract scans the whole app again (it
saw only components/); local/layers refuses an import of an entry (main.tsx, the /watch page) from
any layer, and reads a backtick import() as the path it is; the /watch contract forbids the shell,
the modules and the features by folder (its one path to App had gone stale); chunkGuard's own test
gets the vitest focus/skip rules. The root tests' exemption from local/layers is in the gate
register, with its three imports.

Placement: Plex's chip builder moves into lib beside the feed's, the folder names into
features/folders, and three files only one page uses into that page. ui/FilterRail takes its reset
from useRail instead of a page id. The channel page reads the feed's view and builds "Show in feed"
itself, and the feed's scroll key is the feed's hook, so the shell holds no feed logic.

The floating header re-measures when one page's pills replace another's in the same commit (the
claim count stayed equal, so only the observer caught it). A Plex view applied and edited now stays
across a visit to another page, as the feed's does: pinned by a test. docs/development.md says what a
page with a rail adds in lib.

frontend/src/modules/plex/PlexSidebar.tsx

@@ -8,7 +8,6 @@ import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { ListMusic, Plus, SlidersHorizontal } from "lucide-react";
 import { api, plexFilterCount, type PlexFilters } from "../../lib/api";
 import { notifyActionError } from "../../lib/actionErrors";
-import { type MediaSourceKind } from "../../lib/mediaSource";
 import { useDebounced } from "../../lib/useDebounced";
 import FilterRail from "../../components/ui/FilterRail";
 import { type PanelItem } from "../../components/ui/PanelGroups";
@@ -39,7 +38,7 @@ const DURATION_BUCKETS: { key: string; min: number | null; max: number | null }[
 
 export default function PlexSidebar() {
   // Its layout and collapse flag are its own (lib/useRail), so the shell only places it.
-  const { layout, setLayout, collapsed, onToggleCollapse } = useRail("plex");
+  const { layout, setLayout, onResetLayout, collapsed, onToggleCollapse } = useRail("plex");
   const { t, i18n } = useTranslation();
   // Scope / watch-state / sort / expanded filters live in PlexProvider (shared with PlexBrowse and
   // the header search box); this rail is the primary editor of them. Opening a playlist is a
@@ -51,9 +50,6 @@ export default function PlexSidebar() {
   // They reach the shared views UI in this rail through the scope below.
   const savedViews = usePlexSavedViews();
   const { openModal, modalOpen } = savedViews;
-  // This rail's media source (the Plex library) — names the abstraction the feed and Plex rails
-  // share; drives the panel layout key. `FilterRail` owns the customize `editing` state now.
-  const source: MediaSourceKind = "plex";
   // Titles are display-only (chip labels) — keep them OUT of the facet query key so renaming/relabelling
   // never refetches. collectionMode stays IN the key: any/all changes the result set.
   const { collectionTitles: _cts, ...facetKey } = filters;
@@ -298,7 +294,7 @@ export default function PlexSidebar() {
   return (
     <SavedViewsScope value={savedViews}>
       <FilterRail
-        kind={source}
+        onResetLayout={onResetLayout}
         title={t("sidebar.filters")}
         icon={<SlidersHorizontal className="w-4 h-4" />}
         count={activeCount}